Amazon badly wants to deliver packages of DVDs and Cheez-Its to your doorstep in a matter of minutes—and it wants to use drones to do so. At a NASA convention in July, Amazon Prime Air’s vice president proposed the company’s vision for how unmanned aircraft could one day safely navigate our skies. And NASA recently began testing its first version of an air traffic management system for drones—the agency is partnering with companies including Amazon and Verizon to develop the system.

For now, regulations and technical issues make widespread drone deliveries impossible, which means an army of flying machines probably will not fetch your holiday gifts this year or even the next. Here’s what experts note as the major challenges to resolve before delivery by drone becomes a reality.

Sense-and-avoid technology
Picture a delivery drone flying your package kilometers from its distribution center all the way to your doorstep—technology needs to make every step of this trip safe and reliable. Most importantly, a drone must be able to detect manned aircraft, other drones and any other obstacle (like birds and power lines) to avert potential crashes.

Drones’ “sense and avoid” capacity likely will combine technologies such as cameras, radar and a surveillance system called Automatic Dependent Surveillance–Broadcast (ADS-B), which transmits data about an aircraft’s position and velocity to air traffic controllers and other aircraft. All this should keep drones from, say, colliding with birds or the plane that you’re on. ADS-B technology is now lightweight enough for small drones to carry, but its tracking abilities aren’t very useful until everyone else in the air relies on the system as well. And that might not happen until 2020, which is when the Federal Aviation Administration (FAA) will require most aircraft to be equipped with it.

Not only do delivery drones need to be able to avoid objects at high altitudes—they also have to navigate that last 100 meters to your home, which often presents a veritable maze of power lines, trees, cable dishes, buildings, people—you name it. Parts of this maze can change daily, which makes things even harder for drones—you may have temporarily erected a ladder or put swing set in your yard that were not there during the last delivery. Unmanned aircraft will need to problem-solve those situations on their own, like a Mars rover. “We’ve learned that one of the hardest parts of deploying drones,” says Todd Humphreys, an associate professor of aerospace engineering at The University of Texas at Austin, “is giving them a level of autonomy that would enable them to make decisions when things get complicated in the last tens of meters.”

Humphreys says one solution is that drones could build an instant 3-D map of your yard by merging data from multiple sensors, like cameras and LiDAR (for, light detecting and ranging. This would allow drones to assess their surroundings and independently decide whether anything is in their way. The strategy requires quite a bit of processing power but the technology may be ready for use soon. “I think it’s pretty close,” Humphreys says. “Even with a smartphone-grade processor, you can do this 3-D reconstruction in real time.” His lab is also working on lightweight and inexpensive GPS with subdecimeter accuracy that could help guide drones more precisely.

Fully functioning sense-and-avoid technology already exists today and developers have tried it out in real-world trials, but it is not yet suited for actual deliveries. “The problem is that the full suite of instruments required for sense and avoid is way too heavy, too bulky and too expensive to be practical for Amazon drones,” Humphreys explains.

Low batteries and outmoded regulations
Another big obstacle to drone delivery is battery life. Companies like Amazon want their drones to make up to a 32-kilometer round-trip to drop off packages, but today’s batteries will not last that long. “The small aircraft used to deliver loads less than a couple of pounds are currently limited to a 15- to 20-minute trip,” Humphreys says. “That just doesn’t get you the round-trip that you’d want.” It is possible that vendors may come up with a logistical solution rather than one based on technology, such as driving a van to a neighborhood and releasing drones from there to drop off packages nearby. Those shorter deliveries are possible with current technology, Humphreys says.

But more than anything else, regulations pose the greatest barrier to drone delivery. The FAA effectively bans commercial deliveries, unless it grants an exemption. The agency mandates that the drone operators keep their aircraft within line of sight, which obviously would not work for something like Amazon Prime Air.

Plus, a big part of the reason companies want to deliver with drones is that the machines largely take humans out of the equation. But because the FAA requires all drones have an operator, this makes unmanned aircraft less appealing. “Drone delivery is only cost-efficient if the drone is delivering the package without a person piloting it,” says Gregory McNeal, an expert on drones and a professor of law and public policy at Pepperdine University. “And the FAA presently anticipates that each drone will have its own operator. That’s a 1980s solution to a 2015 problem.” McNeal says there is also the issue of when the FAA will approve sense-and-avoid technology, and the agency has yet to create regulations that establish flight corridors for commercial deliveries, which is what Amazon proposed at the recent NASA convention.

There are a lot of other challenges facing commercial drone delivery, too, such as airspace rights as well as how drones will cope with severe weather and dodge fanatics that try to shoot them down—and of course, there are privacy concerns. But in terms of the engineering, we’re not far from a day when large-scale deliveries are feasible. “We’re right on the verge of being able to do this with current technology,” Humphreys says. Whether Amazon’s winged wares land on your doorstep sooner or later—that’s largely a question for the FAA.